In recent years, as means for forming an electronic circuit, a method of providing a circuit pattern on a plastic board, a ceramic board, or an insulating board coated with plastic or the like, and bonding electronic components such as IC elements, semiconductor chips, resists, and condensers onto the circuit pattern by soldering is widely used.
Among the means, in a method of bonding a lead terminal of the electronic component to a predetermined portion of the circuit pattern, steps of forming a thin solder layer in advance on the surface of a conductive circuit electrode on the board, printing a solder paste or a flux on the thin solder layer, loading and positioning a predetermined electronic component, and melting and solidifying the solder by causing the thin solder layer or the thin solder layer and the solder paste to reflow are sequentially performed in general.
In recent years, along with the miniaturization of electronic products and circuit boards, electronic components are required to have a fine pitch. As the electronic components, for example, a QFP (Quad Flat Package) with a pitch of 0.3 mm, a CSP (Chip Size Package), an FC (Flip Chip) with a pitch of 0.15 mm, and an LSI chip having a BGA structure are known. In addition, as a method of mounting the electronic components on the circuit board, a method of superimposing solder bumps formed in the electronic components on solder bumps formed in the circuit board and causing the bumps to reflow is known. In this method, solder bumps having a fine pattern shape that can respond to the fine pitch of the electronic components are required.
As a method of forming solder bumps on the circuit board, electroplating, non-electrolytic plating, a method of printing a paste of solder powder and causing the paste to reflow, and the like are known. However, in a method of producing the solder bump by the non-electrolytic plating, it is difficult to make the solder layer thick, and in a method of producing the solder bump by the electroplating, it is difficult to apply electric currents for plating to a complex circuit. In addition, in the method of printing the solder paste, it is difficult to respond to a fine pitch pattern. In these circumstances, as a method of forming solder bumps having a constant and uniform height, a method of attaching solder balls to a circuit is used.
As the method of attaching solder balls to a circuit, a method of imparting adhesiveness by allowing an adhesive-imparting compound to react with the surface of a conductive circuit electrode of a circuit board and attaching solder powder to the adhesive layer is known. After this method, the circuit board is heated, whereby solder bumps are formed (see Patent Document 1). In addition, as an application of this method, a technique of attaching only one solder powder particle to a necessary portion has also been developed (see Patent Document 2).